Paranoia has a stigma to it. If you feel that people are working against you there is something wrong with you. There are conspiracies, sometimes they are grand and consist of many conspirators. To believe that you are the object of widespread hatred is not always narcissistic. Learning to recognize camouflaged hatred is an important life-skill.
Somebody hates you (1)
That's without knowing you, too. They hate you for being one of a gender or a race or a religion, or a sexual preference, or the lack of a religion. They might hate you for not being easily identified or for not overtly siding up with them. It doesn't matter who you are or what you do somebody, will hate you.
They hate you, specifically. You. I don't care how lonely you are, how reclusive you are, somebody somewhere has noticed you and hates your guts. Everybody has enemies is not some cynical myth it's true. They hate you because of the way you wear your pants, the fact that you get Chinese delivered one night more a week than they do, the fact that your car is newer, or that you had an Obama yard sign a year ago. Odds are they know your name and that they search through your garbage weekly for some reason to make an anonymous call to the police.
The hatred is profound
It is an expression of any one of a number of issues they have. You are an excuse, a scapegoat, an outlet for all the emotions they have pent up and held inside them. You are the high school teacher who humiliated them in front of the class, the girl who pointed at the tiny penis and laughed, the father who subtly undermined their confidence in their teen years. You are a substitute for all the people they want to hurt and you never know why.
They will likely be sneaky about it
When people hate you for no reason they know that to just come out and tell you or to be overtly hostile would not be well received. They would look like an asshole (possibly racist, sexist, or xenophobic) to everybody and possibly crazy as well. No, they will keep it hidden behind a smile. They will then spend a lot of time looking at you, trying to find a reason they can use to legitimately hate you and to get other people to hate you as well.
You can't read minds
The fact is that there is always a fair chance that any given stranger will be hostile. It's far from a certainty, but the odds are pretty good that they don't like you or your kind. You can gamble that they are not, or you can bet that they are and act accordingly. If you are wrong in the first instance then you might get hurt. Wrong in the second and only they get hurt. Which sounds better to you? Bad people exist. Nobody disputes that. You know that there is always a possibility of evil people working to cause you harm. They exist. If they exist then that guy over there might be one of them. It's possible, if it's possible then you
People are interested
All of them. That's why TV is popular, that's why porn is addictive, it's the joy of getting to see into the private places in other people's lives. It doesn't matter how boring or unattractive you are, if there is a hole in your wall somebody will spend a lot of time on the other side of it just to be able to see you without being seen.